The requirements regarding the precision of machine tools with rotating tools, e.g. milling machines, keep increasing. Apart from the precision of the actual machining as determined by the precision of the machine, the machining tool, the tool holder etc., the precision in setting up the work piece or in setting the zero point for the machining is increasingly turning into a limiting factor for the obtainable precision.
After a work piece has been inserted into a machine and clamped for machining, whether it be by means of pallets or also by conventional means such as a bench vise, the clamping position of the work piece is usually determined with suitable auxiliary devices, and the zero point for machining is determined based on the identified clamping position.
What is known from the state of the art is the probing of the zero point with an automatic probing device. At one end, the latter has a probe tip with a probing ball for probing the work piece. At the other end, it is attached to a tool holder through which it can be interchanged into the machining spindle. For probing a work piece, the machine moves the automatic probing device held at the spindle to the work piece via the tool holder, until the tracer pin that is located at the bottom of the automatic probing device touches the work piece and deflects it so far that the probing device switches. At that, modern automatic probing devices are constructed in such a way that the path for the deflection of the tracer pin is always exactly identical and that it can be calibrated. When the automatic probing device switches, a signal is sent by the automatic probing device to the control, and the latter determined the actual position of the machine for this probing position. In this manner, any edges, bores etc. can be probed, and their relative position in the machine can be determined in a relatively precise manner. Modern probing devices are highly accurate. Therefore, the position of the probing device can be determined for the switch point with a very high precision.
Nonetheless, the precision of this method is limited by the predefined procedure itself. After the predefined geometry elements have been probed in a machine by means of an automatic probing device, the tool holder at which the automatic probing device is attached must be exchanged from the spindle again, and the machining tool that is desired for the machining process must be interchanged into the spindle. Since the clamping position of the tool holders is never exactly identical, deviations occur in the course of this exchange process. In addition, in most cases the length of the automatic probing devices from the lay-on surface of the tool holder up to the tip of the tracer pine does not correspond to the length of the machining tool from the lay-on surface of the tool holder up to the tool tip. Additional deviations occur if the spindle is not positioned exactly in parallel with respect to the displacement direction of the axis that displaces the spindle in the direction of the spindle axis. Even minimal tilting of the spindle leads to an offset between the probing and the machining procedure. This tilting has the more impact on the probing accuracy the greater the length difference between the machining tool and the automatic probing device.